1. Field of the Invention
The present invention relates generally to an optoelectronic receiver, and more particularly to a high speed differential optoelectronic receiver.
2. Description of the Prior Art
Optoelectronic receivers are known in the art. An example of a commercial optoelectronic receiver 10 is shown in FIG. 1. In such receiver two incident complementary amplitude modulated optical signals 12 and 14 are provided to a balanced detector including photodiodes 16 and 18, respectively, and the developed voltages are applied to the single input of an electronic amplifier 20. The optical signals may be either digital or analog. The electronic amplifier 20 converts the signal to an analog voltage level depending on the optical intensity delivered by the input amplitude modulated optical signals. Thus, when more light is delivered from one of the two complementary input signals, the electrical output of the amplifier is increased above the equilibrium level. When there is more light in the other complementary output, the output of the amplifier 20 is decreased below the equilibrium level. As illustrated, the photodiodes 16 and 18 are connected in series in a balanced configuration between the bias voltages +V and -V. The anode of photodiode 16 is connected to the cathode of photodiode 18. Each photodiode has an inherent capacitance associated with it. As will be explained later, this capacitance tends to limit the data rate performance of the receiver. When light 12 is applied to the photodiode 16, a current having a magnitude proportional to the intensity of the light in the first complementary output signal is conducted from the input of the electronic amplifier 20 in a direction as shown by the arrow 22 associated with the photodiode 16. Similarly, when light 14 is applied to the other photodiode 18, a current having a magnitude proportional to the intensity of the light in the second complementary optical signal is conducted in an opposite direction into the input of the electronic amplifier 20 as shown by the arrow 24 associated with the photodiode 18. If the resulting current is applied to the amplifier 20 in a first direction, then its first complementary output signal is greater in intensity. Similarly, if the resulting current is applied to the electronic amplifier 20 in a second direction, then the second complementary output signal is greater in intensity.
As is well known, the data rate of an optoelectronic receiver is limited by the magnitude of the capacitance present at the input of the electronic amplifier. This commercial balanced detector optoelectronic receiver 10 has an effective input capacitance associated with both the photodiodes 16 and 18. As configured, the serially connected diodes present twice the capacitance associated with a single photodiode, and thus unduly decrease the upper data rate achievable. Moreover, the performance of a high speed electronic amplifier 20 depends to some degree on the nature of the input current signal. Current signals that flow in only one direction, either into or out of, the electronic amplifier allow for a more straight forward amplifier design that yields higher performance. Electronic amplifiers designed to accommodate input currents that flow in both directions are more complex and can not easily achieve as high a performance. The conventional balanced detector optical receiver 10 that utilizes anode to cathode connected photodiodes must accommodate input currents that flow in both directions. Thus it is difficult to achieve the highest levels of performance from the electronic amplifier. In addition this receiver cannot utilize a commercially packaged photodiode with an internal resistor termination. Hence, it can not be impedance matched to transmission lines or the like which allow for detection by the photodiodes to occur at some distance from the amplifier. These limitations in the performance of this prior art optoelectronic receiver present difficulties in achieving a high speed optical communication system. Optoelectronic receivers of the type described are sold commercially by New Focus, Inc. of Santa Clara, Calif. as Models 1607 and 1617.
What is needed, therefore, is an optoelectronic receiver which is operable at higher data rates, and which can simplify broadband matching and allow for the photodiodes to be remotely located relative to the receiver amplifiers without compromising its high speed performance.
In addition, it is desirable to require that the high speed electronic amplifier only has to sink or source current at its input, thereby enabling the use of higher performance electronic amplifiers in the optoelectronic receiver.